Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I'm coming from a computer science background, and will be writing some software to automate battery testing. I have background in math, but little to none in physics or electronics. Please consider this in your answers.

That said, how would I go about testing the internal resistance of a small lithium polymer cell by hand?

share|improve this question
up vote 7 down vote accepted

Measure open-circuit voltage \$V_{oc}\$, then load the battery with resistor \$R_{load}\$ and measure voltage \$V_{loaded}\$.

$$ R_{in} = R_{load} \cdot \frac{V_{oc}-V_{loaded}}{V_{loaded}} $$

This is the internal resistance at DC and at your specified load. Internal resistance varies with load and temperature and battery charge and age, etc. And you'll need more advanced equipment to measure at higher frequencies.

share|improve this answer
Internal resistance varies with temperature yes, but I have never heard of it varying with load. Do you have any info of the subject? – Gunnish Jan 10 '13 at 16:08
Everything I read indicates that batteries' internal resistance changes with load. Maybe when you apply a load to the battery, it performs some work thus heating up and thus affecting the IR? – Igor Zevaka Mar 12 '13 at 2:48
Internal resistance is just an approximation. There's not actually a resistor in there. If you take the small-signal response of voltage vs. current, that slope is a resistance. It's not a perfect model. Battery chemistry is complex. – markrages Mar 12 '13 at 3:50

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.